Robert C. Stroop is one of the many obscure Depression-era aeronautical inventors who created innovative if somewhat impractical designs that, for various reasons, never saw their full potential. The author has unable to uncover much about the background of Mr. Stroop apart from a few interesting facts. In surviving correspondence with the Army Air Corps, Stroop claims to have had 20 years of experience in the study, building, flying, and testing of aircraft in major aircraft plants and companies, though he doesn’t name any specific ones. He mentions having worked in metal aircraft manufacturing on the West Coast before moving to the South, the isolation of which had proven to be ideal while developing his designs “undercover in secrecy.” According the excellent Aerofiles.com, Stroop’s first documented design was the SP-4 of circa 1927. Designed in Rome, Georgia, the SP-4 was powered by a 90hp Curtiss OX-5 and could convert from a biplane to a triplane arrangement, a theme that would show up in his later studies. Very little is known about the machine and it may never have been completed. Stroop appears to have subsequently relocated to Jacksonville, Alabama, where he produced the Scout in 1932, a conversion of a Thomas Morse Scout to a parasol monoplane to test his “arcuate” (curved) aileron system, which may be the one depicted in this patent. No information is available regarding the outcome of these tests, which were likely unfavorable. In any event, he appears to have returned to Rome, Georgia, where he conceived the polymorphic SP-6 and SP-7 “Speed Planes,” which are the subject of this article.

The first document in USAAC files concerning the Stroop proposals is a crude hand-written letter to Wright Field dated May 27, 1935 accompanied by three photos of the SP-6 and a general arrangement drawing of the SP-7, a slightly more refined version of the preceding study. In cruising flight, the aircraft functioned as a conventional high speed cantilever wing monoplane, which Stroop claimed could achieve 400-500 mph. For take-off and landing, the wings would split into two halves, the upper part moving upwards and the lower part downwards to create an “X” when viewed from the front. The conversion from monoplane to biplane nearly doubled the wing area. This feature, combined with the heavily cambered lower wing section, allowed for an estimated landing speed of 50 mph. The low landing speed made the aircraft ideal for operation from small airstrips and also made it an ideal platform for pilots with less hours and experience than needed on contemporary fixed wing high speed planes.

According to the scale on the original drawing (3/8 in=1 ft), the SP-7 was 21.9 ft long, had a span of 30.5 ft (as a monoplane), and a height of 11.3 ft (as biplane, horizontal attitude). The inventor stated that “Due to lack of sufficient time, I was unable to complete data necessary for submission of my New Design Speed Plane.” (More likely, Stroop wasn’t sufficient skilled enough to provide the kind of engineering data required by the Materiel Division). He did, however, have a partially completed full-scale mock-up and a wind tunnel model with a 34 inch wing span, both of which featured a working convertible wing that demonstrated the principle of operation behind his invention. He estimated that the model would be ready for wind tunnel testing in about a week to 10 days and was very interested in cooperating with “interested parties” to further develop the design.

The Air Corps’ predictably negative response, prepared by Major W.E. Kepner, Contracting Officer, was sent on June 8, 1935. First, Kepner noted that while extremely ingenious in general arrangement, the detail design of Stroop’s invention would prove to be difficult and would preclude the possibility of developing a usable aircraft for several years, this being independent of any aerodynamic merits or difficulties. Second, due to the pressures of routine work, the Air Corps could not test Stroop’s wind tunnel model. Third, due to the fact that the Air Corps was primarily interested in equipment suitable for procurement in quantity to equip the service, development projects could be undertaken only when the goal and the intermediate steps were apparent. Fourth, Stroop was encouraged to submit his design and model to the Inventions Board of the National Advisory Committee for Aeronautics (NACA). Finally, Kepner stressed that data submitted to the Air Corps in response to Circular Proposals should be capable of immediate reduction to practice.

Nearly 4 years passed before Stroop sent another hand-written letter dated March 10, 1939 to the Army Air Corps, basically reiterating the unique advantages of his unusual X-wing design and his eagerness to gain the government’s support for his project. He references pictures of the mock-up, which was 75% complete at the time of writing, but no photos were found in the file. The Air Corps responded to Mr. Stroop’s submission with a polite form letter on March 16, suggesting that he submit detailed drawings and engineering data directly to the Chief of the Materiel Division at Wright Field in Dayton, Ohio. Thereafter, there is no further correspondence concerning the project. However, Aerofiles.com lists the SP-7 as having been tested in the wind tunnel in 1940, but probably never actually built and flown; hopefully further research will shed light on the matter. If any readers have more information on Robert C. Stroop or his designs, they are encouraged to comment below or email the editor.

There is some passing similarity between the Stroop designs and the Nikitin-Schyevchyenko IS series of polymorphic fighters built between 1938-41 in the Soviet Union; the latter actually made it to flight test, where it was discovered that the performance of the monoplane configuration was inferior to that of more conventional monoplane fighters; this was likely due to the added weight and complexity of the folding wing mechanism. The Stroop Speed Plane likely would have had the same problem had it actually taken to the air.

All images from NARA Archives II, College Park, MD, RG 342

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